Conventional systems, as described in Research Applications Report Vol. 28, No. 7, p. 210, Chapter 3.1.1, are known to be of three types including (1) fixed delayed insertion-removal plus frame memory, (2) two-frame memory, and (3) elastic store plus frame memory.
All of these three types of system are such that the phase of writing data into a frame aligner memory (hereinafter referred to as "the frame memory") is compared with the phase of reading the same data, and the approach of one phase to the other is detected thereby to control the writing into the frame memory.
In a system for transmitting low-speed signals having a frame structure within a high-speed frame in multiplex fashion, it is necessary to assure the time sequence (hereinafter referred to as "TSSI", an abbreviation of Time Slot Sequence Integrity) by matching the phases between the low-speed signals arriving from the same channel at the time of terminating the high-speed frame. FIG. 4 shows a case in which the TSSI is assured, and FIG. 5 shows a case in which it is not assured. In FIGS. 4 and 5, there are two types of low-speed frames, A and B. The numerical value in each frame indicates the order of generation of the particular frame. The received frames are aligned in accordance with a predetermined reference phase at the receiving end. Now, assume that the TSSIs of A and B are required to coincide with each other at the receiving end, i.e., that the TSSIs of A and B are required to be assured. In such a case, the reference phase must exist at a place where the order of generation of A coincides with that of B as shown in FIG. 4.
All the conventional systems described above handle only a single frame, so that in terminating the above-mentioned high-speed frames, it is required that each low-speed signal be separated and be subjected to frame alignment individually. As a result, as shown in FIG. 8, in the case where a plurality of low-speed signals to be terminated arrive from a plurality of source stations with the phases thereof distributed over a whole frame, an attempt to assure the TSSI for each of all the source stations would fail as it is impossible to set a reference phase capable of assuring the TSSI for all the source stations.